Datasheet

AD9125
Rev. 0 | Page 32 of 56
DIGITAL DATAPATH
The block diagram in Figure 50 shows the functionality of the
digital datapath. The digital processing includes a premodulation
block, three half-band interpolation filters, a quadrature modulator
with a fine resolution NCO, a phase and offset adjustment
block, and an inverse sinc filter.
PREMOD
PHASE
AND
OFFSET
ADJUST
HB1 HB2 HB3
SINC
–1
09016-020
Figure 50. Block Diagram of Digital Datapath
The digital datapath accepts I and Q data streams and processes
them as a quadrature data stream. The signal processing blocks can
be used when the input data stream is represented as complex data.
The datapath can be used to process an input data stream
representing two independent real data streams as well, but
the functionality is somewhat restricted. The premodulation
block can be used, as well as any of the nonshifted interpolation
filter modes (see the Premodulation section for more details).
PREMODULATION
The half-band interpolation filters have selectable pass bands
that allow the center frequencies to be moved in increments of
½ of their input data rate. The premodulation block provides a
digital upconversion of the incoming waveform by ½ of the
incoming data rate, f
DATA
. This can be used to frequency-shift
baseband input data to the center of the interpolation filters’
pass band.
INTERPOLATION FILTERS
The transmit path contains three interpolation filters. Each of
the three interpolation filters provides a 2× increase in output data
rate. The half-band (HB) filters can be individually bypassed or
cascaded to provide 1×, 2×, 4×, or 8× interpolation ratios. Each
of the half-band filter stages offers a different combination of
bandwidths and operating modes.
The bandwidth of the three half-band filters with respect to the
data rate at the filter input is as follows:
• Bandwidth of HB1 = 0.8 × f
IN1
• Bandwidth of HB2 = 0.5 × f
IN2
• Bandwidth of HB3 = 0.4 × f
IN3
The usable bandwidth is defined as the frequency over which
the filters have a pass-band ripple of less than ±0.001 dB and an
image rejection of greater than +85 dB. As is discussed in the
Half-Band Filter 1 (HB1) section, the image rejection usually sets
the usable bandwidth of the filter, not the pass-band flatness.
The half-band filters operate in several modes, providing
programmable pass-band center frequencies as well as signal
modulation. The HB1 filter has four modes of operation, and
the HB2 and HB3 filters each have eight modes of operation.
Half-Band Filter 1 (HB1)
HB1 has four modes of operation, as shown in Figure 51. The
shape of the filter response is identical in each of the four modes.
The four modes are distinguished by two factors: the filter center
frequency and whether the input signal is modulated by the filter.
0
–20
–40
–60
–80
–100
021.81.61.41.21.00.80.60.40.2
GAIN (dB)
NORMALIZED FREQUENCY (× f
IN1
)
.0
MODE 0
MODE 1 MODE 3
MODE 2
09016-021
Figure 51. HB1 Filter Modes
As shown in Figure 51, the center frequency in each mode is
offset by ½ the input data rate (f
IN1
) of the filter. Mode 0 and
Mode 1 do not modulate the input signal. Mode 2 and Mode 3
modulate the input signal by f
IN1
. When HB1 operates in Mode 0
and Mode 2, the I and Q paths operate independently and no
mixing of the data between channels occurs. When HB1 operates
in Mode 1 and Mode 3, mixing of the data between the I and Q
paths occurs; therefore, the data input into the filter is assumed
complex. Table 16 summarizes the HB1 modes.
Table 16. HB1 Filter Mode Summary
Mode f
CENTER
f
MOD
Input Data
0 DC None Real or complex
1 f
IN
/2 None Complex
2 f
IN
f
IN
Real or complex
3 3f
IN
/2 f
IN
Complex